In the past, typical conventional boilers were high water content boilers that were not designed to condense their flue gas. These non-condensing boilers operated most efficiently at high percentage output capacities. These non-condensing boilers also were limited to minimum water temperatures to prevent condensing; otherwise the boiler life would be shortened by corrosion. Therefore high (or full) capacity output together with high system temperature conditions is the preferred operating mode to achieve greater efficiencies while operating within the physical operating limits of these boilers.
A new type of high efficiency low water content boilers has been introduced into the market in recent years. These high efficiency boilers are referred to as condensing boilers because they encourage flue gas condensing which results in greater heat transfer and therefore greater operating efficiencies. However, the operating efficiency of these condensing boilers varies greatly as a function of boiler output capacity and system water temperatures. These boilers also have different physical operating limitations than conventional non-condensing boilers. These boilers are intended to operate at lower temperatures and may have high operating temperature limitations (stack temperatures or water temperatures). In summary, these boilers operate more efficiently at low percentage output capacities and low water temperatures (where condensing can occur).
The most common control system for hydronic heating systems in the market today is an indoor-outdoor reset control systems that computes the required system water temperature as a function of outdoor temperature (See FIG. 1). As the outdoor temperature decreases the desired system water temperature is increased and the control creates a demand signal for greater boiler plant capacity to achieve the higher system water temperature. Correspondingly, high outdoor temperatures directly correlate to lower system water temperatures and less boiler capacity.
Such reset control systems do not distinguish output demand signals based upon these different types of boilers. For this reason, there has been great difficulty in mixing condensing and non-condensing boilers in the same boiler plant system to take advantage of the best efficiency operating characteristics and operating physical limitations of each type of boiler because these indoor outdoor reset systems dynamically change the required system water temperature requirement.
Therefore, there is a need for a control system that will factor these different operating efficiencies and operating physical limitations into account when determining which type of boilers to operate in a mixed condensing/non-condensing multiple boiler system that uses indoor outdoor temperature reset of the system water temperature. Since high efficiency condensing boilers also are generally more expensive than non-condensing boilers, equipment capital costs can be reduced while maintaining overall heating plant efficiency by installing fewer condensing boilers to only provide a portion of boiler plant capacity at low system water temperatures, while the installed non-condensing boilers can be capable of providing 100% of the boiler plant capacity at the high system water temperatures (see FIGS. 2A and 2B). In some instances, manufacturers automatically switch to non-condensing boilers in colder weather, and use condensing boilers to achieve maximum load. Generally, in such systems high system water temperatures require greater boiler plant capacity to achieve maximum performances.
Prior art systems include that in U.S. Pat. No. 4,864,972 “Boiler Optimization for Multiple Boiler Plants” which optimizes multiple boiler selection based upon measured efficiencies at different outdoor temperatures. That system does not account for different types of boilers and the effect of different system temperatures on their efficiency or physical operating limits that results when used indoor-outdoor reset logic is applied to control system water temperature to such multiple boiler systems.